1. Field of the Invention
The present invention relates to a resistor composition, and more particularly, to a lead-free, ruthenium-based thick film resistor composition used to form a thick film resistor in, for example, various types of resistor components such as chip resistors, semi-fixed resistors, variable resistors, focus resistors, and surge devices, as well as in thick film circuits, multilayer circuit substrates and various types of laminated composite components.
2. Description of the Related Art
Thick film resistor compositions are mainly composed of an electrically conductive component and glass, and are used to form thick film resistors on various insulating substrates. Resistor compositions are printed in a predetermined shape on an alumina substrate, ceramic composite component or the like, having electrodes thereon, in the form of a paste or coating material, and are fired at a high temperature of about 600 to 900° C. After subsequently forming a protective coating with an overcoat glass as necessary, the resistance value is adjusted by laser trimming and so on as necessary.
Examples of characteristics required of resistors include a small temperature coefficient of resistance (TCR), low current noise, satisfactory withstand voltage characteristics, ESD characteristics and long life characteristics, and satisfactory process stability, or in other words, small changes in resistance values caused by process fluctuations. In addition, in the case of chip resistors in particular, adjustment of the resistance value by laser trimming is required to be able to be carried out easily, and deterioration of characteristics after trimming is required to be small.
Resistor compositions have widely been used in the prior art which use a ruthenium-based oxide powder for the electrically conductive component. These ruthenium-based resistor compositions can be fired in air, and by changing the ratio of electrically conductive component and glass, resistors can be easily obtained having a wide range of resistance values from 1 Ω/□ or less to several MΩ/□, while also demonstrating satisfactory electrical characteristics and superior stability.
Examples of substances used as electrically conductive components of ruthenium-based resistor compositions mainly include ruthenium dioxide, ruthenium composite oxides of pyrochlore structure such as bismuth ruthenate, lead ruthenate, etc., ruthenium composite oxides of perovskite structure such as barium ruthenate, calcium ruthenate, etc., and ruthenium precursors such as ruthenium resinates. In particular, ruthenium composite oxides are preferably used in high-resistance resistor compositions having a high glass content. This is because, since ruthenium composite oxides normally have a higher resistivity than ruthenium dioxide by a factor of 10 or more, they can be blended in larger amounts than ruthenium dioxides, thereby reducing variations in resistance values, and allowing the obtaining of stable resistors having satisfactory current noise characteristics, TCR and other resistor characteristics.
In addition, glass —containing lead oxide was primarily used for the glass. This is because lead oxide-containing glass has superior characteristics suitable for the formation of thick film resistors, including a low softening point, satisfactory fluidity and good wetability to an electrically conductive component, superior adhesion to substrates, and a coefficient of thermal expansion that is compatible with ceramics, and particularly alumina substrates.
However, lead components are toxic and are undesirable from the perspective of their effects on the human body and pollution. Amidst the requirements placed on electronics products to comply with WEEE (Waste Electrical and Electronic Equipment) and RoHS (Restriction of the Use of Certain Hazardous Substances) requirements in order to deal with environmental issues in recent years, there is a strong need to develop lead-free materials for resistor compositions as well.
Several resistor compositions have been proposed in the past which use lead-free ruthenium dioxide, bismuth ruthenate, alkaline earth metal ruthenates or the like as electrically conductive components and lead-free glass (see Patent Documents 1 and 2, listed below).
However, resistor compositions have not yet been obtained which do not use lead glass but are still comparable to conventional lead glass-containing ruthenium-based resistor compositions, demonstrating superior characteristics over a wide range of resistance values. It has been particularly difficult to form high-resistance resistors having a resistance of 100 kΩ/□ or more.
In general, many of the ruthenium composite oxides used in high resistance ranges tend to decompose to ruthenium dioxide having a lower resistivity than ruthenium composite oxides as a result of reacting with glass during firing of the resistor composition at high temperatures. In the case of combining with lead-free glass in particular, it is difficult to inhibit the decomposition to ruthenium dioxide during firing in the vicinity of 800 to 900° C. Consequently, the resistance values decreased and thereby prevented the obtaining of a desired high resistance value, while also resulting in the problems of an increased film thickness dependency and firing temperature dependency. The decomposition can be inhibited to a certain degree by using a ruthenium composite oxide powder having a large particle size such as a mean particle size of 1 μm or more as in Patent Document 1. However, the use of a coarse electrically conductive powder worsens current noise and load —life characteristics, thereby preventing the obtaining of superior resistor characteristics.
In addition, in the case of combining with lead-free glass, ruthenium composite oxides are typically considered to have a difficulty in producing fired films having a satisfactory microstructure. For example, although bismuth glass like that described in Patent Document 2 is known to be effective for preventing decomposition of bismuth ruthenate, this resistor composition has a large negative TCR in the high resistance range and cannot be used in such applications.
Observation of the microstructure of a resistor fired film with an electron microscope normally reveals extremely fine conductive particles dispersed throughout the glass matrix, with the conductive particles contacting each other to form a network (network structure). This is thought to result in the formation of conductive pathways, thereby demonstrating. electrical conductivity. In the case of resistor compositions using conventional ruthenium composite oxides and lead-free glass, it is extremely difficult to form a stable network structure (to be referred to as a “conductive network”) in the high resistance range, especially where the ratio of the electrically conductive component is low. Consequently, it has been difficult to produce lead-free resistors demonstrating high resistance values and having superior TCR characteristics and current noise characteristics as well as minimized fluctuation of the characteristics.
[Patent Document 1] Japanese Patent Publication No. 2005-129806A
[Patent Document 2] Japanese Patent Publication No. 8-253342A